Method for etching aluminum



Dec. 30, 1969 s c T 3,486,954

METHOD FOR ETCHING ALUMINUM Filed Feb. '7, 1966 ALKALJ ETCH SOLUTIONETCH SOLUTION ALKALI SILICATE.

ADDITION l4 REGEN-E'QATED ALKALI ETCH soLJJTloN AND PIZECIPITATEDALUMINOSILICAT'E ALUN'HNOSlLICATE PRECIPITATE James H. HSHCQflF- TINVENTOR.

syj w United States Patent METHOD FOR ETCHING ALUMINUM James A.Ashcraft, Manhattan Beach, 'Calif., assignor, by

mesne assignments, to McDonnell Douglas Corporation, Santa Monica,Calif., a corporation of Maryland Filed Feb. 7, 1966, Ser. No. 525,328

Int. Cl. C23f 1/04; B08b 3/08; C01d 1/32 U.S. Cl. 15619 6 ClaimsABSTRACT OF THE DISCLOSURE Regeneration of alkali depleted aqueousalkali etching solutions, e.g., sodium hydroxide, and precipitation ofaluminate as alumiuosilicate by the addition of an alkali metalsilicate, e.g., sodium silicate.

This invention relates to a method for etching aluminum, and is moreparticularly concerned with a procedure for regenerating or rejuvenatingsolutions employed in the etching of aluminum.

The term aluminum as employed herein is intended to denote eithersubstantially pure aluminum or any of the alloys of aluminum.

For the etching of aluminum, particularly to obtain a deep etch, forexample, as practiced in the aircraft industry, it is known to employhot alkali, usually sodium hydroxide, solutions for this purpose, andthese solutions may contain additives such as sulfur orsulfur-containing compounds in the form, for example, of sodiumpolysulfide, to improve the etching operation or t e properties of theetched aluminum surfrace which is produced.

In such etching operations, the alkali, e.g., sodium hydroxide, solutionreacts with the aluminum surface to form sodium aluminate in knownmanner. After a period of etching, a substantial amount of such sodiumaluminate is formed in the etching solution, and the alkali content ofthe solution is materially reduced. These conditions substantiallyreduce the effectiveness of the alkali etching solution for etching thealuminum surface and also adversely affect the quality of the etchobtained.

In large measure, it has been the practice heretofore to dump or discardspent alkali etching solutions containing substantial amounts of sodiumaluminate, and to employ a fresh alkali solution. This practicesubstantially increases the cost of these operations. However, in orderto regenerate such solutions and to overcome the effects of theaccumulation of sodium aluminate in aluminum etching solutions and toremove or avoid the formation of excess amounts of sodium aluminate,various prior art procedures have been developed. Thus, for example, inUS. Patent 2,975,041 spent etching solution containing alkali metalaluminate is cooled in the presence of solid hydrated alumina toprecipitate an additional quantity of solid hydrated alumina, toregenerate the solution and increase the alkali metal hydroxide contentof the solution, followed gy separating the precipitated hydratedalumina. However, this procedure is time consuming and requires asubstantial amount of equipment to carry out the procedure. In US.Patent 2,650,875, additives such as sodium gluconate are added to thealkali etching solution so as to prevent the reversion of the sodiumaluminate to sodium hydroxide and alumina, or by controlling thisreversion to produce a non-adherent alumina precipitate. However, toapplicants knowledge, there is lacking in the prior art any teaching ofa simple direct inexpensive method in the chemical etching of aluminumemploying alkali etching solutions such as sodium hydroxide, forregenerating the spent etching solution by simultaneously readilyremoving excess amounts of sodium aluminate from and increasing thealkali content of such solution, so as to provide an efiicient reuseablealkali etching solution for aluminum.

According to the invention, it has been found that alkali etchingsolutions for aluminum, e.g., caustic soda, can be simply andeconomically regenerated or rejuvenated by the addition of alkali metalsilicate, preferably sodium silicate, to the spent etching solutioncontaining alkali metal, e.g., sodium, aluminate. The sodium silicatereacts with the sodium aluminate to precipitate out the aluminate and toregenerate alkali, to thereby permit reuse of the solution for aluminumetching. The precipitates so formed is an aluminosilicate which tends toagglomerate in the form of small balls that fall to the bottom of thesolution and can be removed.

The etching treatment for aluminum is generally carried out in a hotaqueous solution containing an alkali such as sodium or potassiumhydroxide, trisodium phosphase, sodium carbonate, or the like, ormixtures thereof. Preferably, aqueous sodium hydroxide solutions areemployed. Temperature of the solution during the etching process mayrange say from about room temperature up to about 212 F. Generally, ahot solution is employed with temperature maintained in the range fromabout F. to about 210 F. during treatment. The alkali concentration ofthe etching solution can vary and such concentration may range fromabout 0.1 to about 10 normal, e.g., in the range from about 1 to about 3normal.

In order to improve etching results, e.g., for producing etchedsurfraces which are smooth and fine grained, various additives can beincorporated in the etching solution. Thus, for example, asulfur-containing additive such as an alkali metal sulfide, e.g., sodiumsulfide, an alkali metal polysulfide such as sodium polysulfide, orsulfur can be incorporated in minor amounts, e.g., from about 0.1 toabout 40%, by weight of the alkali, to improve the characteristics ofthe etched aluminum surface as noted above. However, it is understoodthat such additives are optional and need not be present in the alkalietching solution.

As previously noted, during the etching operation aluminum is dissolvedfrom the aluminum body being etched and forms alkali metal aluminate insolution, e.g., sodium aluminate. As the etching reaction proceeds, theconcentration of sodium aluminate progressively increases. The presenceof small or moderate concentrations of sodium aluminate in the etchingsolution does not adversely effect to any appreciable extent the rate ofetching or the quality of the etched surface. As a matter of fact, ithas been found that it is preferable to maintain the presence of somealuminate in the etching solution during the etching process, providedthe amount of such aluminate does not become deleterious from thestandpoint of substantially reducing the etching rate or degrad ing thequality of the etched aluminum surface obtained. Thus, it has been foundthat the presence of sodium aluminate in small or moderateconcentrations, e.g., not greater than about 6 ounces, preferablybetween about 3 and about 5 ounces, of sodium aluminate calculated as A10 per gallon of solution, is desirable in order to obtain a good highquality etch and to prevent intergranular pitting, islanding, bad edges,and non-uniform surface of the etch.

Thus, when the amount of alkali metal aluminate, e.g., sodium aluminate,formed during the etching process increases to a point where the qualityof the etch is reduced, and/or the etch rate substantially decreases,according to the invention alkali metal silicate, e.g., sodium orpotassium silicate, is added to the etching solution, preferably in anamount to precipitate sufiicient of the aluminate so as to reduce thealkali aluminate concentration in the etching solution to an amountpreferably not greater than about 6 ounces of aluminate, calculated asA1 0 per gallon of solution, and more desirably between about 3 andabout 5 ounces of such aluminate, calculated as A1 per gallon. For thispurpose, the alkali silicate, e.g., sodium silicate, added to theetching solution, is preferably in the form of an aqueous solution suchas a solution of water glass, which is a commercially available 37%solution of sodium silicate. In preferred practice, and to produce analuminosilicate precipitate which does not tend to adhere to the tankwalls and which readily drops to the bottom of the tank for easy removaltherefrom, relatively dilute sodium silicate solutions are employed,e.g., sodium silicate solutions having a concentration of about 3 toabout 10%, e.g., about 5%, sodium silicate. Such solutions can bereadily provided by suitably diluting the commercially available 37%water glass (sodium silicate) solution.

Generally, about 0.1 to about ounces of alkali metal silicate, e.g.,sodium silicate, can be employed, per gallon of spent alkali etchingsolution containing alkali aluminate, and usually about 0.5 to about 2ounces of such alkali metal silicate is employed, per gallon of spentetch solution. Hence, the aqueous sodium silicate solution preferablyemployed is utilized in amounts sutficient generally to provide alkalimetal, i.e., sodium, silicate in the above ranges. Larger amounts ofalkali metal silicate within the above ranges are employed where theamount of aluminate in the solution is relatively large, e.g.,substantially above 6 ounces per gallon of aluminate, calculated as A1 0Smaller amounts of alkali metal silicate within the above ranges areemployed where the amount of aluminate in solution is relatively small,e.g., of the order of about 6 ounces per gallon of aluminate or somewhatlower or higher. It will be understood that the alkali metal silicatecan be added at intervals to the alkali etching solution during a periodof etching, or the silicate added following a period of etching, toreduce the aluminate concentration and to increase the alkaliconcentration by regeneration, to desired levels.

The reaction between the aluminate present in the spent alkali etchingsolution, and the alkali silicate produces a white aluminosilicateprecipitate, usually in the form of small diameter balls, e.g., of theorder of about /8 inch in diameter, which tend to drop to the bottom ofthe etching vessel. The exact composition of the aluminosilicateprecipitate formed is not known to me, but is believed to be of acomplex nature. As an example, a sample of such a complexaluminosilicate precipitated from a spent alkali etching solutionaccording to the invention contained about 44.5% A1 0 and about 26.3%SiO the balance being largely water of crystallization. However, it isto be understood that the term aluminosilicate employed herein isintended to designate the precipitated reaction product of the reactionof alkali metal, e.g., sodium, aluminate and alkali metal, e.g., sodium,silicate, produced by the addition of such silicate to a spent etchingsolution containing such aluminate, according to the invention,regardless of the exact chemical composition of such aluminosilicate.

The aluminosilicate precipitate thus formed in the etching solutionduring regeneration thereof according to the invention can be readilyremoved by any suitable means, such as by mechanical means, or bycentrifuging, or by siphoning out a portion of the spent etchingsolution containing such aluminosilicate precipitate. Simultaneouslywith removal of the aluminosilicate precipitate, small amounts of anyother precipitate such as copper sulfide, zinc sulfide and magnesiumhydroxide which may be formed during the etching process, where thecorresponding metals are present, e.g., as alloying constituents in thealuminum being etched, and the etching solution contains asulfur-bearing additive, such as sulfur per se, can also be removed.

Upon addition of the alkali silicate, e.g., sodium silicate, to thespent alkali etching solution containing sodium aluminate, at least aportion of the alkali, e.g., sodium hydroxide, consumed in the etchingreaction is regenerated simultaneously .with production andprecipitation of the aluminosilicate. A typical reaction mechanism isset forth below but is not intended as limitative of the invention:

Thus, by the procedure according to the invention, not only is theexcess aluminate removed from the solution in the form ofaluminosilicate precipitate, but additional alkali is produced toincrease the alkali concentration of the spent solution. This permitsthe regenerated solution to be reused for further chemical etching ofaluminum, without addition of fresh alkali, although if desired, somefresh alkali can be added to increase the alkali content to a desiredconcentration.

It is noteworthy that the addition of alkali silicate, e.g., sodiumsilicate, to a spent chemical etching solution including the above notedsulfur or sulfur-containing additive such as sulfur, sodium sulfide orsodium polysulfite, does not adversely affect the function of suchadditives when the regenerated solution is again employed for etching analuminum body.

The flow diagram shown in the accompanying drawing illustrates theprocess of the invention whereby an alkali etching solution, indicatedat 10, which becomes alkali depleted after a period of aluminum etching,as indicated at 12, is regenerated by addition of alkali silicate whichalso simultaneously causes precipitation of aluminosilicate, asindicated at 14. The regenerated etch solution is then further used toetch aluminum.

The following are examples of practice of the invention.

EXAMPLE 1 A 7075 aluminum body is etched in a hot alkali etchingsolution maintained at about 190 F. The initial etching solution has thefollowing composition.

Ounces Components: per gallon Sodium hydroxide 21.3 Sulfur 7 The etchingreaction takes place for a period of about 30 minutes, after which theetching solution contains a concentration of sodium aluminate equivalentto about 7 ounces A1 0 per gallon of solution and the causticconcentration is reduced to 17.2 ounces of sodium hydroxide per gallonof solution. At this point, the rate of the etching reaction issubstantially decreased and the quality of the etch is visiblydiminished, e.g., as to smoothness of etch, and other important etchproperties noted above.

To the black hot spent etching solution is added 1.2 ounces of sodiumsilicate, as a 37% sodium silicate solution, commercially available asWater glass, per gallon of etching solution. A white precipitate formsin the nature of small balls which drop to the bottom of the etchingvessel. Such precipitate is readily removed from the etching tank andthe regenerated alkali etching solution is found to contain thefollowing proportions of sodium hydroxide and sodium aluminate:

Ounces Components: per gallon Sodium hydroxide 18.6 Sodium aluminatecalculated as A1 0 4.43 Specific gravity 1.1932 at 25 C.

5. minum to produce a high quality etched aluminum surface at goodetching rates.

EXAMPLE 2 The procedure of Example 1 is carried out except that the 37%solution of sodium silicate employed is diluted seven times so that theconcentration of sodium silicate in the silicate solution added to thespent alkali solution is about 5%. I

It is observed that the aluminosilicate precipitate formed afteraddition of the diluted sodium silicate solution to the spent alkalietching solution, more efifectively drops to the bottom of the tank andsubstantially none of such precipitate adheres to the side walls of thetank. Substantially all of the aluminosilicate precipitate, which islocated at the bottom of the tank, is readily removed leavingpractically no residue of aluminosilicate scale on the tank walls.

EXAMPLE 3 Pieces of 2024 and 7075 aluminum are etched in a hot aqueoussodium hydroxide etching solution at about 185 F. until the alkalicontent of the solution is reduced to 15.6 ounces sodium hydroxide pergallon of solution and the alkali aluminate content has reached about 9ounces of sodium aluminate, calculated as A1 per gallon of solution.

Sodium silicate (in the form of water glass) is then added to the alkalietching solution in an amount of about 2 ounces of sodium silicate pergallon of etch solution. A precipitate of aluminosilicate forms, whichis removed from the etching tank. The concentration of aluminate in theetching solution is thus reduced to about 5 ounces per gallon, and thealkali concentration is increased to about 17.3 ounces per gallon ofsolution.

The regenerated solution is employed to etch additional pieces of 2024and 7075 aluminum at good etch rates and with production of a goodquality etch on the aluminum parts.

From the foregoing, it is seen that the invention provides a simple andeconomical procedure for readily and rapidly regenerating orrejuvenating spent alkali etching solutions, by addition of alkalisilicate to precipitate out excess aluminate in the form ofaluminosilicate, and to increase the alkali concentration of the spentsolution, and permit its reuse for etching additional aluminum bodies.

While I have described particular embodiments of my invention for thepurpose of illustration, it should be understood that variousmodifications and adaptations thereof may be made within the spirit ofthe invention, as set forth in the appended claims.

I claim:

1. A method of etching aluminum which comprises contacting said aluminumwith a hot aqueous alkali solution for a period sufiicient to etch saidaluminum and to form alkali metal aluminate in said solution andreducing the concentration of said alkali, and adding an alkali metalsilicate to the resulting etching solution to precipitate aluminate inthe form of an aluminosilicate, and to regenerate alkali in saidsolution.

2. A method as defined in claim 1, said alkali solution being an aqueoussodium hydroxide solution, said alkali metal aluminate being sodiumaluminate, and said alkali metal silicate being in the form of anaqueous sodium silicate solution.

3. A method as defined in claim 2, the sodium aluminate content of saidetching solution following said period of etching being greater thanabout 6 ounces calculated as A1 0 per gallon of solution, and the amountof aqueous sodium silicate solution added to said etching solution beingsufficient to precipitate an amount of aluminosilicate to reduce theconcentration of the sodium aluminate in said etching solution to notgreater than about 6 ounces calculated as A1 0 per gallon of saidsolution.

4. A method of etching aluminum as defined in claim 1, which includescontacting said aluminum with a hot aqueous sodium hydroxide etchingsolution for a period to etch said aluminum and to form sodium aluminatein said solution and reducing the concentration of sodium hydroxide,adding an aqueous sodium silicate solution to the resulting etchingsolution in an amount sufiicient to provide from about 0.1 to about 10ounces sodium silicate per gallon of etching solution, to precipitatealuminate in the form of an aluminosilicate, and to increase the sodiumhydroxide concentration of said solution, and reducing the concentrationof sodium aluminate to not greater than about 6 ounces calculated as A10 per gallon, removing said precipitated aluminosilicate, and contactingaluminum with the separated sodium hydroxide solution to etch said lastmentioned aluminum.

5. A method as defined in claim 4, the amount of sodium silicatesolution employed furnishing about 0.5 to about 2 ounces of sodiumsilicate per gallon of etching solution.

6. A method as defined in claim 4, said sodiu-m' hydroxide solutionbeing a 0.1 to about 10 normal solution, and said sodium silicatesolution having a concentration of about 3 to about 10% sodium silicate.

References Cited UNITED STATES PATENTS 9/ 1953 Dvorkovitz et al 15 6-222/ 1943 Hagelin 156-22 OTHER REFERENCES JACOB H. STEINBERG, PrimaryExaminer US. Cl. X.R.

